Plasma levels of immunoreactive prostaglandin E were measured in 22 patients with essential hypertension and 18 normotensive subjects after equilibration on 109 mEq and 9 mEq/day sodium intake. Mean levels of immunoreactive prostaglandin E were similar in hypertensive and normotensive subjects and did not change with altered dietary sodium. The findings do not support the hypothesis that decreased levels of the vasodepressor prostaglandin, prostaglandin E, are a factor in the genesis of essential hypertension.